Why Does `dict.fromkeys()` Create Shared Lists, and How Can I Avoid It?

Fromkeys Caveats: Understanding the Behavior of Dictionary Initialization

In Python, the dict.fromkeys() method is commonly used to create a dictionary with specified keys assigned to empty lists. However, misinterpretation of its behavior has led to unexpected results.

Why fromkeys() Is Tricky

When an empty list [] is provided as the second argument to dict.fromkeys(), all values in the resulting dictionary point to the same list object. This means that any modification to one value affects all values.

Alternative Solutions

To address this issue, the following solutions are recommended:

• Use a Dictionary Comprehension:

  In Python 2.7 or above, a dictionary comprehension is the most concise and readable solution:

  data = {k: [] for k in range(2)}

• Use a List Comprehension with dict():

  In earlier Python versions, a list comprehension can be passed to the dict() constructor:

  data = dict([(k, []) for k in range(2)])

• Use a Generator Expression with dict():

  In Python 2.4-2.6, a generator expression can be used with dict(), and parentheses can be omitted:

  data = dict((k, []) for k in range(2))

Example

The following example illustrates the difference between the default behavior of fromkeys() and the recommended solutions:

# Default behavior of fromkeys()
data = {}
data = data.fromkeys(range(2), [])
data[1].append('hello')
print(data)  # Output: {0: ['hello'], 1: ['hello']}

# Using a dictionary comprehension
data = {k: [] for k in range(2)}
data[1].append('hello')
print(data)  # Output: {0: [], 1: ['hello']}

By understanding the nuances of fromkeys(), developers can effectively create dictionaries of empty lists and maintain individual key addressing.

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